Genetic architecture of hybrid fitness and wood quality traits in a wide interspecific cross of eucalyptus tree species

by 1969- Myburg, Alexander Andrew

Abstract (Summary)

MYBURG, ALEXANDER ANDREW. Genetic Architecture of Hybrid Fitness and
Wood Quality Traits in a Wide Interspecific Cross of Eucalyptus Tree Species. (Under the
supervision of Ross W. Whetten and Ronald R. Sederoff.)
The genetic architecture of interspecific differentiation plays a key role in the evolution of
reproductive isolating barriers in plants, and has important implications for hybrid breeding
programs. Interspecific hybridization is an important approach towards the genetic improvement
of Eucalyptus tree plantations. However, incompatibilities between diverged eucalypt genomes
often lead to reduced fitness of interspecific hybrids and decrease the efficiency of hybrid
breeding programs. Furthermore, very little information is available on the genetic control of
quantitative traits in interspecific hybrids of Eucalyptus tree species.
The aim of this project was to obtain a detailed description of the genetic architecture of
hybrid fitness and wood property traits in an interspecific cross between two commercially
important hardwood tree species, E. grandis and E. globulus spp. globulus. This cross combines
the superior growth and adaptability of E. grandis with the excellent wood properties of E.
globulus. The two species are members of different sections of the subgenus Symphyomyrtus and
hybrid generations of this wide cross are characterized by large amounts of F1 hybrid inviability
and F2 hybrid breakdown.
Two interspecific backcross families of E. grandis × E. globulus were each genotyped with
more than 800 amplified fragment length polymorphism (AFLP) markers using a new highthroughput
protocol for AFLP analysis on automated DNA sequencers. A pseudo-backcross
mapping approach was used to generate detailed comparative genetic maps of a single superior F1
hybrid tree and of two backcross parents. The genetic maps of the two pure species parents and
the F1 hybrid were colinear and 11 comparative synteny groups were characterized. Overall
recombination rates did not differ significantly, although heterogeneity in recombination rates
was observed in several map regions.
Approximately 30% of the AFLP markers were significantly distorted from expected
segregation ratios. The distorted markers were located in specific map regions and distortion was
highly directional in these regions. A Bayesian Markov chain Monte Carlo approach was used to
estimate the position and effect of genetic factors that cause segregation distortion. At least six
segregation distorting loci (SDL) were located in the genetic maps of the F1 hybrid. Two SDL
were detected in the genome of the E. grandis backcross parent and one SDL in the map of the E.
globulus backcross parent. Donor alleles were found to be favored in the recurrent genetic
background at several SDL in the maps of the F1 hybrid. Marker-assisted breeding based on
detailed, whole-genome genotypes of hybrids may be useful to minimize the effect of hybrid
breakdown factors in further generations of this cross.
Near-infrared (NIR) analysis was used to predict wood property trait values for approximately
270 individuals of each backcross family after two years of growth in a field site. The trait data
and AFLP genotypes were used for QTL detection and analysis in the parental genetic maps. In
addition, a principal component analysis was performed on the NIR spectral data of each
backcross family and the data used to map QTLs for NIR spectral variation. A total of 18 QTLs
for NIR predicted wood properties and NIR spectral variation were characterized in the parental
maps of the E. globulus BC family, while 13 QTLs were detected in the E. grandis BC family.
Many of the QTLs had effects on multiple, correlated wood property traits and on raw NIR
spectral variation. Individual principal components of NIR spectral variation were correlated with
groups of chemical and physical wood properties, and shared several large-effect QTLs with the
individual wood properties. These QTL may represent key genetic loci that are involved in the
genetic differentiation of wood properties between these two species.
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